I am always looking for new calming strategies to teach children. One of my favorites is the Take 5 Breathing. This technique is from the Managing Big Emotions series for kids at childhood101.com. A youtube video can be found here.
I find this calming strategy beneficial because, in addition to deep breathing, it also lets children focus their eyes and attention on their hands. I have found that letting them do so increases their ability to cope with all of those overwhelming emotions. Tip: it is VERY common for kids to try to move too quickly through their fingers, which makes this method less effective. I have to give plenty of prompts to slow down and they will need lots of practice with this one.